Gasoline DISC engine technology has the potential of improving fuel economy through the use of stratified combustion, which significantly extends the lean burn limit and reduces pumping losses in the engine. Compared with a conventional port fuel injection (PFI) gasoline engine, a DISC engine is much more complicated in its hardware and operating strategy. Like a PFI engine, a DISC engine consists of an intake manifold, combustion chambers, and an exhaust system. Its hardware design and configuration, however, are different from a PFI engine in several key aspects. The location of injectors is different. In a DISC engine, fuel is injected directly into the cylinder as opposed to the intake port. The fueling system also differs. A high pressure fueling system is an important aspect of the DISC technology and is operated at a pressure that is 10-15 times higher than that of a PFI fueling system. The combustion chamber configuration of DISC engines also include non-flat piston heads having deliberately designed cavities to ensure charge stratification. The after-treatment package of a DISC engine typically requires the combination of a three-way catalyst (TWC) and a lean NO.sub.x trap (LNT) to meet emission standards.
With the special piston design and the high pressure fueling system, a DISC engine can effect two distinct modes of operation by properly timing the fuel injection in relation to other engine events. By injecting early in the intake stroke, there is enough time for the mixing of air and fuel to form a homogeneous charge by the time the ignition event is initiated. On the other hand, by injecting late in the compression stroke, the special combustion chamber design and the piston motion will lead to the formation of a stratified charge mixture that is overall very lean, but rich around the spark plug. In a typical DISC engine, a properly positioned swirl control valve can also contribute to enforcing the stratification in one mode and assuring good mixing in another.
The torque and emission characteristics corresponding to these two modes are so distinct that different strategies are required to optimize the engine performance in these different modes. Furthermore, in addition to the standard engine control variables such as throttle, fueling rate, spark timing and exhaust gas recirculation (EGR), other inputs, such as injection timing, fuel rail pressure and swirl control valve setting are also available. In addition, like other lean burn technology engines, DISC engines require special exhaust gas treatment systems to meet emissions regulations. For example, the lean NO.sub.x trap (LNT), which represents a state-of-the-art technology for NO.sub.x reduction for lean burn engines, has a narrow temperature window and stringent air-fuel ratio control requirements. It also has to be periodically purged to regenerate its trapping capacity and maintain its efficiency. A DISC engine running in stratified mode typically requires the LNT to be purged by running slightly rich of stoichiometry for 2-3 seconds at about 50 second intervals. Managing the engine torque, as well as the LNT operating temperature during this purge cycle is important to maintaining driveability and system efficiency.
The increased system complexity, coupled with more stringent fuel economy and emissions requirements, has made the DISC engine a control-intensive technology which depends heavily on the control system to deliver its expected benefits. Given the multitude of control inputs and performance indices, such as fuel consumption, emissions and other driveability measures, the DISC engine control strategy development and system optimization rely heavily on model-based approaches and computer aided control design tools. Thus, there exists a need for a DISC engine model structure which encompasses both homogeneous and stratified mode of operation.
In addition, the engine control system must manage the LNT purge cycle described above without causing noticeable torque disturbance to the vehicle.